This application is based on an application No. 10-163400/1998 filed in Japan, the content of which is incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to an image forming apparatus which forms a color image by superposing images of a plurality of colors formed by a plurality of print units.
2. Description of Prior Art
In a color image forming apparatus of tandem structure, an image of four colors is formed at the same time in a scan. In the color image forming apparatus, four print units including photoconductors for the four print colors are aligned, and images of the four colors are supplied to exposure positions for each color, by delaying in correspondence to the distances between the photoconductors. In a system wherein four beams are scanned with a polygon mirror, the direction of luster scan becomes reverse to each other between the former two colors and the latter two colors, and the data are corrected therefor. Further, color shifts are caused by various factors.
Print positions for the four colors may be shifted due to image delay by the memories and the shift of exposure positions. Then, color displacement occurs, and image is distorted. When such image distortion is prevented, an image of high quality can be obtained. Then, in order to correct the shift of print positions, it is proposed to detect image distortion quantities and to change the line address in the main scan direction for the colors for an address in the main scan direction according to the detection results.
However, the result of edge decision cannot be corrected when the line address in the main scan direction for the colors for an address in the main scan direction is changed according to the detection results of image distortion. Therefore, noises happen in the image to be outputted.
An object of the present invention is to provide an image forming apparatus having an image distortion correction system which can perform edge processing more correctly.
In one aspect of the invention, an image forming apparatus includes an image forming unit including a plurality of print units each forming an image of one of a plurality of colors based on image data for one of a plurality of colors. A color image is formed by the print units each forms an image of a color different from each other. An input section receives the image data and discrimination signals of character edge. A detector detects distortion of image, and a first corrector circuit corrects the image data. Further, a second corrector circuit corrects the discrimination signal of character edge according to the image distortion detected by said detector, and a controller controls the image forming unit to form a color image according to the image data corrected by the first corrector circuit and the discrimination signal of character edge corrected by the second corrector circuit.
In another aspect of the invention, in an image processing apparatus, an input section receives image data of a plurality of colors of a color image and attribute discrimination signals which represent image attributes. A detector detects shifts of images of the plurality of colors, and a first corrector circuit changes the image data to correct the shifts of the images of the colors according to the shifts detected by the detector. Further, a second corrector circuit corrects the attribute discrimination signals according to the shifts detected by said detector.
An advantage of the present invention is that an image of higher quality can be formed according to correct edge information.
Another advantage of the present invention is that the edge decision can be corrected with a small-scale circuit by correcting the edge decision in synchronization of image distortion correction in the main scan and subscan directions.